1. Field of the Invention
The invention is a means to correct for color shifts and inaccurate color rendition when using dual video or electronic image projectors to project stereoscopic images using the polarized light method of image selection. The invention is applicable to projectors which inadvertently produce polarized light as a result of their image forming methods. Projectors which use mirror surfaces, semi-silvered or metalized surfaces, dichroic filters, or liquid crystal (LC) display, will produce polarized light and will thus require the corrective means described in this disclosure.
2. Description of the Related Art
Stereoscopic film and video projection systems produce stereoscopic images, which are commonly known as "3-D" images. In one class of conventional projection system, a "3-D" image is produced when two synchronized projectors project polarized left and right images onto the same projection screen. Throughout this disclosure, including in the claims, the expression "synchronized projector" (or "synchronized projection system") denotes a projection system which is controlled to project a left perspective (or right perspective) image simultaneously with projection (by another projection system) of a corresponding right perspective (or left perspective) image, where the images projected by the two projection systems together represent a stereoscopic image.
As shown in FIG. 1, the polarized image of each projector (1, 2) is created by passing a non-polarized image, created by a traditional film projecting system, through a polarizing filter (3 or 4). The orientation of the axes of the polarizing filter for the projectors are typically orthogonal to each other; commonly at 45.degree. and 135.degree., respectively, from a vertical line. The "3-D" effect appears when a viewer wears spectacles which have polarized lenses (5 and 6) that correspond to the polarization axes of the polarizing filters.
Recently, stereoscopic video projection systems have become an important display technique in several applications, notably in flight simulators and in the design of automobiles. Stereoscopic video systems operate on the same principle as do stereoscopic film systems except that a film system uses a film for the image source while a video system uses a video source for the image source.
One stereoscopic video production system which utilizes a single projector is the field-sequential approach, as described by Lipton in "Field-Sequential Electronic Stereoscopic Projector," SPIE Vol. 1081, 1989. One problem with the field sequential approach, however, is that a special short persistence green phosphor cathode ray tube (CRT) must be used. The problem with these special CRTs, which typically cost several times the price of a normal green CRT, is that the brightness level is significantly reduced.
Thus, a need exists for a stereoscopic video projection system which is not dependent on the special CRT.
Another type of video projection system utilizes dual projectors, each with a liquid crystal (LC) image forming panel (rather than a single projection having a CRT display). One problem with using a single projector LC projection system with the field sequential approach is that, in the field sequential approach, the projector must run at twice the video field rate. Conventional LC projectors, however, cannot run at that rate because the crystal switching speed is too slow. Thus if LC projection systems are to be successfully used for stereoscopic or three-dimensional projection, it must be by means of the dual projector approach.
One problem with using the LC display projection system in the traditional dual projector approach is that the LC system produces an image with polarized light as an artifact of the image forming means. When projecting a non-stereo image, a polarized video image is of no consequence because the polarization does nothing to degrade the image since the eye is insensitive to polarized light. However, if two polarized images are passed through polarizing filters in the traditional dual projector approach, a severe color shift of the stereoscopic image will result. A further problem is that the brightness of the stereoscopic image may be reduced.
Another problem with using the LC display projection system in the traditional dual projector approach is that the degree of color shift from each projector is different. Such color shifts are unpleasant to view--blue skies may appear to be red, skin tones blue, green grass purple, and so on. Such bizarre skin tones and strange colors of objects which are familiar to the viewer are annoying and intolerable.
Therefore, there is a need to provide a stereoscopic video projection system that would reduce color shift and illumination reduction problems which occur in conventional stereoscopic video projection systems.